The present invention relates to a twin fan turbomachine with two contrarotating fans and more particularly to a special arrangement of the elements that connect the fans to their drive shafts.
In known turbomachines that incorporate a twin fan mechanical design, two fan impellers spaced axially apart are rotationally driven in contradirections by shafts of a low-pressure turbine located in the downstream part of the turbomachine.
Each low-pressure turbine shaft is attached at its upstream end to a fan impeller drive shaft. The upstream fan impeller drive shaft is connected to the downstream part of the upstream impeller and is surrounded by the drive shaft that drives the downstream fan impeller, which shaft is connected to the upstream part of the downstream fan impeller by a frustoconical connecting wall.
The downstream fan drive shaft is guided at its upstream end by an upstream bearing which is positioned radially inside the downstream fan impeller, and at its downstream end by a downstream bearing. These two bearings are connected by mounts to turbomachine casings. The upstream end of the upstream fan shaft is connected to the upstream end of the downstream fan shaft by an upstream inter-shafts bearing, its downstream end being connected to the downstream end of the downstream fan shaft by a downstream inter-shafts bearing. These two inter-shafts bearings center the upstream fan drive shaft inside the downstream fan drive shaft.
This type of twin fan design has the advantage of reducing the unwanted noise emitted by the engine by halving the rotational speed of each of the two fans by comparison with a single-fan engine and also allows turbomachine efficiency to be improved. However, turbomachines with contrarotating fans known in the prior art do have a number of disadvantages.
Specifically, when the turbomachine is running, the way in which the downstream fan impeller is attached to its drive shaft does not allow the impeller to be kept optimally centered under the effect of centrifugal force. What is more, the mount supporting the upstream bearing that guides the downstream fan shaft needs to be rigid enough to be able in particular to withstand the loads transmitted through the upstream and downstream fan impellers which means, given how long it is, that use has to be made of a relatively heavy mount.
Superimposing the upstream guide bearing with the upstream inter-shafts bearing means that any change to the dimensions of one of the two bearings have to be accompanied by a change in the geometry of the other bearing. In addition, such changes are restricted by the proximity with the upstream guide bearing mount to the downstream fan impeller balancing weights which extend toward the inside of the turbomachine.
The guide bearings are lubricated by nozzles fixed on the mounts supporting these bearings. The upstream and downstream inter-shafts bearings cannot be lubricated in this way because these bearings are located within two rotating shafts thus precluding the fitting of nozzles.
One solution is to install a centrifugal scoop on the downstream fan drive shaft, this scoop being supplied with oil by a nozzle fixed to the downstream guide bearing mount. The oil collected by this scoop passes through orifices in the wall of the downstream fan drive shaft. Under the effect of centrifugal force, this oil flows along the frustoconical internal wall of the downstream fan drive shaft and thus lubricates the downstream inter-shafts bearing and the upstream inter-shafts bearing in turn. This solution has proven to be less effective than the solution involving nozzles because a plug of oil may form at the inter-shafts bearings, thus causing the rolling elements of the bearings to slide rather than roll, thus accelerating bearing wear. However, this solution is still advantageous to a twin fan turbomachine because the rotational speed of each fan is only half that of the shaft of a single-fan turbomachine.
Using the internal wall of the downstream fan drive shaft to guide the oil from the downstream inter-shafts bearing toward the upstream inter-shafts bearing prevents ventilation holes from being made in this wall, the disadvantage of this being to create a substantially hermetic enclosed space between the two drive shafts, and to lead to a pressure difference across the downstream fan. There may also be increases in temperature within this enclosed space which also cause the various components and the lubricating oil to heat up.
Oil separation means are commonly used to recover some of the suspended oil from the spaces between the bearings and thus limit engine oil consumption and atmospheric pollution. However, the radial proximity of the upstream and downstream fan drive shafts means that conventional oil separation means, of the type involving radial vents attached to the upstream fan drive shaft cannot be used in the inter-shafts space because correct operation of such means entails there being a minimum distance between the free end of the radial vents and the internal wall of the downstream fan shaft. Recourse is therefore had to means that are more complicated, more expensive and more cumbersome. In addition, the fact that it is not possible to make holes in the wall of the downstream fan drive shaft between the inter-shafts bearings also prevents any oil separation system from being installed between the upstream guide bearing mount and the downstream fan drive shaft.